1. Field of the Invention
This invention relates to resin impregnated fibrous webs and more particularly to resin impregnated fibrous webs having a uniform density throughout.
2. Description of the Prior Art
Resin impregnated sheet materials such as cloth, batts, waterleaves, and the like are well known in the art. These resin impregnated sheet materials are useful for a plurality of purposes including imatitation leather in the form of vinyls and the like, structural sheet materials such as conveyor belts and similar products.
Prior art methods of impregnating a particular web involve the impregnation or coating of a porous material with a polymeric resins such as a polyurethane, vinyl or a similar material. Polyurethanes have met with wide acceptance as a coating or impregnating composition due to their capability of wide variation in chemical and physical properties, particularly their flexibility and chemical resistance. In impregnating the porous sheet material with a polymeric resin several techniques have been employed. One such prior art method involves the use of the polymeric resin in an organic solvent system wherein the sheet material is dipped in the solution and the solvent is removed therefrom. These solvent systems are undesirable since the solvent, in many cases, is toxic and must either be recovered for reuse or discarded. These solvent systems are expensive and do not necessarily provide a desirable product since upon evaporation of the solvent from the impregnated porous sheet material the resin tends to migrate to provide a non-homogeneous impregnation of the porous sheet material resulting in resin richness toward the surface of the sheet material rather than uniform impregnation.
In order to alleviate the problems with solvent systems, certain aqueous polymeric systems have been proposed. In forming impregnated sheet materials by impregnation with aqueous polymers the aqueous portion must be removed. Again heat is required and migration of the polymer to the surfaces of the impregnated sheet material is encountered.
In one method of combining polyurethane solutions with porous substrates the polymer is applied in an organic solvent to a substrate, such as a needle punched polyester batt. The polymer-substrate composite is subsequently bathed with a mixture of organic solvent for the polymer and a non-solvent for the polymer that is at least partially miscible with the solvent until the layer is coagulated into a cellular structure of interconnected micropores. The solvent is removed from the coating layer along with the non-solvent to produce a solvent free microporous layer. Although this process yields acceptable properties for a polyurethane impregnated fabric, it has the disadvantage of an organic solvent system particularly when high performance polyurethanes are utilized which require relatively toxic and high boiling solvents. An example of this method is disclosed in U.S. Pat. No. 3,208,875.
In another method, polyurethane dispersions in organic vehicles have been proposed and used to coat porous substrates such as is disclosed in U.S. Pat. No. 3,100,721. In this system, a dispersion is applied to a substrate, and coagulated by further addition of a non-solvent. Although this approach has been used with some success, it involves two major limitations: (1) the vehicle of the dispersion is substantially organic since relatively small amounts of non-solvent, preferably water, are needed to form a dispersion; and (2) there is a narrow useful range of added non-solvent so that reproducible results are difficult to obtain.
One particularly useful method of preparing composite sheet material by impregnating a porous substrate is disclosed in U.S. Pat. No. 4,171,391 incorporated herein by reference. In this system a porous sheet material is impregnated with an aqueous ionic dispersion of a polyurethane and the impregnant is coagulated therein. The composite is then dried to form a composite sheet material. The present invention is an improvement over this basic process and in some instances is broader in scope.
In accordance with the present invention, a method of impregnating porous sheet materials and particularly needled batts is disclosed wherein uniform impregnation is provided in an aqueous system forming a product with high tear strength and integrity.
Further, an impregnated fibrous web is provided which has a novel and unusual useful structure adapted to be used as formed or subsequently processed to provide further advantages.